As is known in the art, optical techniques for surface profilometry are commonly performed using interferometric measurements. Analyzing the interference fringe pattern formed by overlap of a reflected wave from an optically smooth surface with a reference wave, enables surface profile measurements with high accuracy. Projecting an interference fringe pattern on an object surface is effective for probing rough surfaces. High-resolution, point-by-point measurements of rough surfaces have been demonstrated using a long coherence length source with a Fizeau interferometer and with a broadband source.
White-light interferometry is capable of simultaneously imaging large field of views by scanning only the path length of a reference arm. In this approach, light reflected from the surface interferes with a reference wave to form a speckle pattern on a camera. When the reference optical path length is scanned, each individual speckle exhibits an intensity modulation. The surface height is determined at the maximum point of the modulation envelope. White-light interferometry is an extremely robust technique, allowing for high resolution imaging in three dimensions with a large field of view.
Depth resolved imaging with a large, three-dimensional field of view is more challenging when utilizing small diameter flexible imaging probes such as borescopes, laparoscopes, and endoscopes. Confocal imaging through a fiber-bundle using a lens with a high numerical aperture is one solution to this problem. The three-dimensional field of view for these devices, however, is limited to less than a few millimeters due to the small objective lens clear aperture and low f-number required for high-resolution optical sectioning.
Other methods, such as stereo imaging and structured illumination have been proposed. These methods all require additional hardware for the probe, increasing the size, cost, and complexity of these devices.